The batteries in hybrid cars now get recharged slightly whenever the driver taps the brakes. If research at Honda pans out, heat from the engines could do the same thing.

The Japanese auto giant has released a paper detailing how a Rankine cycle co-generation unit could help recharge the battery in a hybrid and thereby increase gas mileage, according to Green Car Congress. Honda put the Rankine unit in a test car (a Honda Stream) and found that the unit generated more electricity than regenerative braking. However, the unit isn't very efficient so more work will be required before Honda can put one of these in cars.

Waste heat, according to some, is one of the untapped sources of power in the world. Some have proposed harnessing the waste heat from nuclear plants to run water purification systems or produce hydrogen.

The tough part is that it's not easy. Paul Marcoux, vice president of green engineering at Cisco Systems, was recently asked if computer companies could harvest heat from processors and hard drives and turn that into power. Probably not, he said. The temperature generally doesn't get hot enough.

In a Rankine unit, a water pump keeps water under high pressure. Heat from the gas engine in a hybrid is then captured, compressed, and used to make steam out of the water. The steam then turns a generator to make electricity, which charges the battery that runs the electric motor.

Hybrids have two motors: one gas, one electric. In conventional hybrids, the electric motor powers the car around town while the gas motor does more of the work on the freeway. General Motors and Tesla Motors are building cars in which the gas motor doesn't drive the car at all, but runs a generator which charges the battery for the electric motor. Conceivably, a Rankine system could be used in either but would probably work better in a conventional hybrid because the gas engine is larger.

Right now, Honda's Rankine unit is only about 13 percent efficient.

Honda is also trying to bring efficient, cleaner, high-mileage diesels to the United States.